1. Field of the Invention
This invention generally relates to data reproducing techniques, and particularly to a technique capable of stably reproducing data with being less subject to the defects of optical disks.
2. Description of the Prior Art
The construction and operation of a conventional optical disk apparatus will be described first with reference to FIGS. 2 through 6. FIG. 2 shows the construction of part of a conventional optical disk apparatus. FIG. 3A shows the action of a polarizing beam splitter 3 on an inputted laser beam a1, and FIG. 3B shows the action of the polarizing beam splitter 3 on an inputted laser beam b1. FIG. 4A shows the action of a quarter wave plate 4 on the inputted laser beam a1, and FIG. 4B shows the action of the quarter wave plate 4 on the inputted laser beam b1. FIG. 5A shows a method of detecting a bright defect by an envelope detection circuit 9, FIG. 5B a method of detecting a dark defect by the envelope detection circuit 9, and FIG. 5C a method of detecting an amplitude reduction by the envelope detection circuit 9. FIG. 6 shows a method of detecting an abnormal laser-beam emission by a laser power detection circuit 11.
Referring to FIG. 2, there are shown a laser-beam source 1, a collimator lens 2, the polarizing beam splitter 3, the quarter wave plate 4, an objective lens 5, an optical disk 6, a convex lens 7, a reproducing photo detector 8, the envelope detection circuit 9, an irradiation beam photo detector 10, the laser power detection circuit 11, a servo control circuit 20 and a laser drive circuit 21.
The polarizing beam splitter 3 shown in FIG. 2 has two prisms glued slant-face to slant-face with one of their slant faces coated with a polarizing film to form a cube. This splitter 3 acts so that it allows the beam component a1 (P-polarized component) parallel to the x-axis to pass through it as illustrated in FIG. 3A, and that it allows the beam component b1 (S-polarized component) parallel to the y-axis to be reflected as a beam component cl as illustrated in FIG. 3B. The quarter wave plate 4 acts so that it converts the beam component a1 (P-polarized component) parallel to the x-axis into a circular-polarized beam component a2 as shown in FIG. 4A, and that it converts a circular-polarized beam component b2 into the beam component b/(S-polarized component) parallel to the y-axis as shown in FIG. 4B.
The laser drive circuit 20 drives the laser-beam source 1 to irradiate the laser beam a toward the optical disk 6. The laser beam a irradiated from the laser-beam source 1 is converted into a parallel pencil of rays (parallel beam) by the collimator lens 2. Since the laser beam a irradiated from the laser-beam source 1 through the collimator lens 2 is the P-polarized component, the polarizing beam splitter 3 allows this inputted beam to pass as it is. The quarter wave plate 4 converts the incident laser beam a from the polarizing beam splitter 3 into a circular-polarized beam. The circular-polarized laser beam a from the quarter wave plate 4 is focused to hit the rotating optical disk 6 by the objective lens 5.
The laser beam L reflected from the rotating optical disk 6 is collected by the objective lens 5 and is again incident to the quarter wave plate 4. The circular-polarized laser beam b incident to the quarter wave plate 4 is converted into the S-polarized component that is perpendicular to the laser beam a irradiated from the laser-beam source 1. The S-polarized component b is fed to the polarizing beam splitter 3. Since the polarizing beam splitter 3 acts to reflect the S-polarized component as described above, the laser beam b is almost reflected from the polarizing beam splitter 3 into the convex lens 7 as a laser beam c. This laser beam c is fed through the convex lens 7 to the surface of the reproducing photo detector 8. The rest of the beam b is passed as a beam d through the collimator lens 2 and incident to the laser-beam source 1.
The reproducing photo detector 8 is composed of a plurality of light-sensitive elements and an arithmetic circuit. The arithmetic circuit computes the amounts of beam received by those respective light-sensitive elements, and produces a servo error signal e according to the focus position and tracking position of the laser beam a irradiated on the optical disk 6, and a reflected-beam detection signal k according to the absolute amount of the reflected beam. The reflected-beam detection signal k is supplied to a signal processing circuit not shown, where it is processed according to a certain method so that data can be reproduced.
A servo control signal z produced from the servo control circuit 20 can control the objective lens 5 to drive so that the inputted servo error signal e can always be kept at a predetermined value, thus adjusting the focus position and tracking position of the laser beam a. However, when the optical disk 6 has a defect, the servo error signal e changes even when the above focus position and tracking position are optimum. At this time, when the servo control circuit 20 erroneously operates by trying to change the servo error signal e into a certain value, a long time will be sometimes taken until the servo control circuit returns to the normal controlling state even when the spot of the laser beam a crosses over the defect of the optical disk 6 and then scans the correct area of the disk. Therefore, the data will be interrupted when it is reproduced by the above signal processing circuit.
Meanwhile, the envelope detection circuit 9 detects the envelope of the reflected-beam detection signal k fed at its input end and uses slice levels of f1, f2, f3 and f4 as, for example, shown in FIG. 5 to detect the amplitude reduction of the signal k due to the bright defect, dark defect and contamination that occur on the optical disk 6.
In FIGS. 5A through 5C, the abscissas are the time, and the ordinates are the signal k as the amount of the reflected beam and the output signals h1, h2 and h3 produced from the envelope detection circuit 9. In the case of bright defect, the amount of the reflected beam is increased over the normal value because the reflecting film is, for example, exposed at the area where the evaporation on the recording surface of optical disk 6 is inadequate.
The slice level f1 for detecting the bright defect is set at an arbitrary position relative to the upper envelope g1 of the signal k as shown in FIG. 5A. When the upper envelope g1 and the lower envelope g2 exceed the slice level f1 at the bright defect j1, the output signal h1 takes a voltage of H1.
In the case of dark defect, the amount of the reflected beam is decreased to be less than the normal value because dust particles or the like that block the beam are, for example, attached on the surface of optical disk 6.
The slice level f2 is set at an arbitrary position relative to the lower envelope g2 of the signal k as shown in FIG. 5B. When the upper envelope g1 and lower envelope g2 are decreased less than the slice level f2 at the dark defect j2, the output signal h2 takes a voltage of H1.
In the rewritable-type optical disk such as CD-RW, the amplitude of the signal k produced from the detector 8 when the disk is recorded or reproduced is reduced, for example, due to the semitransparent dust attached to the surface of disk 6. The slice levels f3 and f4 for detecting the amplitude reduction are set at arbitrary positions between the upper envelope g1 and the lower envelope g2 as shown in FIG. 5C. The output signal h3 takes a voltage of H1 when the upper envelope g1 and lower envelope g2 are changed to have values between these slice levels g1, g2 at the amplitude-reduction time interval j3 due to the dust on the disk 6. Therefore, when the defects are detected, the servo error signal e is masked by using the above the output signals h1, h2 and h3 to hold the servo control circuit 20 in the off state, thereby preventing the above erroneous operation from being caused due to the above defects.
On the other hand, the irradiation beam photo detector 10 is composed of a beam-sensitive element and an amplifying circuit. This detector 10 receives part of the laser beam a emitted from the laser-beam source 1 and produces an amount-of-irradiation-beam detection signal m according to the amount of the received beam. The laser power detection circuit 11 detects an abnormal beam emission of the laser-beam source 1 in accordance with the inputted signal m as shown in FIG. 6.
In FIG. 6, the abscissas are the time, and the ordinates are the signal m as the amount of part of the beam a and the voltage of an output signal p from the laser power detection circuit 11. In the laser power detection circuit 11, a slice level n1 for detecting the abnormal emission of the laser-beam source 1 is set above the normal beam detection level m1 that is produced when the inputted signal m is reproduced. When the signal m increases over the slice level n1, the output signal p from the laser power circuit 11 takes a voltage of H1 to stop the beam emission of the laser-beam source 1. Thus, the recorded data on the disk can be prevented from being damaged by the abnormal beam emission of the laser-beam source 1 that occurs chiefly in the RW-type optical disk.
In the detection of defects on the disk, it is necessary to stably detect without detecting errors. In this prior art, there is a technique in which the reproducing photo detector 8 is used to more precisely detect the defects by changing the slice levels for the defects in accordance with the variation of the DC component of the amount of the reflected beam (for example, see JP-A-07-141702).